In most of the known information carriers, such as magnetic and optical disks, tapes, cards, etc., the stored information is distributed within a surface of the carrier. The capacity of a device of this class (i.e. two-dimensional memory device) is limited by the surface area and is inverse proportional to the second order of reading radiation wavelength.
There is increasing demand for cheap and reliable large capacity carriers of digital information for computers, video systems, multimedia etc., and for high-density data storage in optical media, particularly for CD-ROM data and documents and image/movie storage in CD-sized disks. Such a carrier should have a storage capacity in excess of 1010 bytes, fast access time, high transfer rate and long term stability. Optical methods of recording and reading out information have advantages over magnetic methods due to less restricted requirements of the components and environment, and ability for parallel recording of information which is advantageous for mass production of such carriers.
There are two ways of increasing the storage capacity of an optical information carrier. One approach is based on the fact that the shorter the wavelength of recording radiation, the smaller the size of the illuminated spot. Hence, by decreasing the wavelength λ of the recording radiation, the density of the stored data can be increased. The storage capacity of an optical disc is diffraction-limited by a value of N bits, wherein N=Disc-area/λ2, because only one binary value is stored in a diffraction-limited pixel. Quadrupled capacity can be gained using “super resolution” at fractions of wavelengths. High density of information is received when 3–5 bits are stored in a single data region, as a small variation of the length of the data region around the diffraction limit. This method requires precision optical, mechanical and electronic components, as well as high quality media, and therefore its capacity is limited by cost effectiveness.
Another approach of increasing the storage capacity of digital data carrier is based on making stacks of two disks. This approach suffers from the following drawbacks:    (1) data regions are light reflective, resulting in undesirable multiple reflection when reading out the stored information;    (2) power losses at each information disk during the propagation of reading and reflected beams to and from the internal disks, respectively;    (3) interference of beams reflected from different disks;    (4) diffraction of beams passing through the disk;    (5) beam distortions due to the optical aberrations, which appear when changing the optical path of the reading beam within the carrier to read different information planes (i.e. different disks); and    (6) high quality optical adhesives required for assembling the stack of disks, having no aberrations, bubbles, separations, inclusions, as well as no mechanical, thermal and chemical impact on the discs.
The information capacity of a stacked information carrier is limited in practice to 1010 bytes. One example of such an information carrier is the known digital versatile disk (DVD) in the form of a stack composed of two information disks. The disks are attached together by back-sides to double the capacity of the carrier.
Yet another approach consists of making a three-dimensional distribution of data regions within an information carrier, i.e. a three-dimensional optical memory device. The capacity of a three-dimensional memory device is proportional to the third order of reading radiation wavelength. The volume distribution of stored information significantly increases the storage capacity, as compared to that of the two-dimensional device. For example, the total thickness of a three-dimensional optical memory device can be about 1 mm and can consist of information layers having thickness of 0.01 mm. Thus, the storage capacity of this device is 100 times greater than the capacity of a single layer.
It is understood that the more information layers, the greater storage capacity of the memory device. However, the maximum number of information layers depends on a suitable reading technique to be used for reading out the stored information. On the other hand, the reading techniques are based on the main principles of the construction of the optical memory device.
A three-dimensional information carrier and a reading device therefor are disclosed, for example, in. U.S. Pat. No. 4,090,031. The information carrier comprises a substrate and a plurality of data layers provided on one side of the substrate. Each of the layers comprises data tracks formed of lines of data spots. The data spots are formed of either binary coded digital information or frequency or pulse length modulated analog information, which is photographically recorded. According to one approach disclosed in the above patent, the data spots are light reflective, being formed of light reflecting metal material having a reflecting index different from that of the layers. Selection of one data track for reading is accomplished by changing the focus of a reading light beam from one data layer to another. The main drawback of this approach is unavoidable multiple reflection and diffraction produced by different layers, resulting in the undesirable crosstalk affecting the signal-to-noise ratio. Practically, for that reason, such a “reflective” three-dimensional information carrier cannot be formed with more than two-three information layers. In other words, information recorded in a “reflective” information carrier is too limited. By an alternative approach, making the data spots of different photoluminescent materials having different optical properties has been proposed. In this case, the illumination means includes a suitable source of “white” light of many frequencies to illuminate different layers by reading beams of different wavelengths. The detection means includes different colored filters accommodated in front of numerous detectors, each associated with a corresponding one of the data layers. It is evident that this technique significantly complicates the manufacture of both the information carrier and the reading device used therewith.
Another three-dimensional information carrier is disclosed in U.S. Pat. No. 5,268,862, wherein a fluorescent material having special properties is utilized as an active, data-containing material. More specifically, the active material contains photocromic molecules having two isomeric forms. The first isomeric form “A” is not fluorescent, it has absorption bands for ultraviolet radiation, and is transferred to the second form “B” under two visible photons absorption. The form “B” absorbs the two photons of reading radiation and fluoresces in the infrared range. A two-photon absorption process is used for writing information into the medium. Two focused beams are crossed at the region having dimensions of λ3, each beam being formed by a picosecond or femtosecond pulse of light to provide the intensity required for both writing and reading processes. This means that two pulses should overlap in time domain. Accordingly, this approach has also a series of drawbacks, which will hardly permit it to be practically realized. First, the two-photon approach requires extremely high intensity laser pulses, I˜1012–1013 W/cm2, which in turn requires the femtosecond pulsewidth Ti:Sapphire lasers. Second, the μm-sized intersection of two focused laser beams required for reading out-the stored information would be very difficult or even impossible for practical realization. Third, the reliable, stable photochrome material which may withstand multiple writing/erasing/reading cycles at a room temperature and possess the optical properties compatible with the existed miniature (diode) laser sources does not yet exist. Another problem is a long time period required for writing the information into the disc, which is about 105 sec, if optimistic information writing rate is 106 bits/sec. This makes the solution proposed in the patent to be very expensive even for mass production.